Janus was a real Roman god. There is no counterpart for Janus in Greek mythology. He was Roman. His nickname is Two-Faced Janus because the Romans pictured him as having two faces, one looking forward, one looking backwards. On coins, he was pictured in profile, one face turned to the left, one face turned to the right, with one neck and one top of his head - one head, two faces.
Janus had a very important job in the ancient Roman world. All Roman gods had a job to do, but Janus was the Roman god of the threadhold of a doorway or gate. His job was to keep evil spirits out of homes, buildings, shrines, schools, courtyards, and wherever there was a doorway or gate. There were a great many doorways and gates in ancient Rome. The people knew Janus was a busy god, keeping evil out of many places. The Roman people took a minute each day to pray to Janus and thank him for doing his part to keep their home safe from evil.
Janus was also part of the Roman new year celebration. The month of January, the first month of the year, was named after him. Each year, on the first day of the month of January, people exchanged presents in his honor.